1. Field of the Invention
The present invention relates to an apparatus and a method for converting power from a power input to an DC output voltage or current, which apparatus comprises a serial resonance converter containing at least two serial coupled semiconductor switches having a common output terminal connected to at least one first coil which coil can be a part of a transformer having a second winding connected to a rectifier means, which rectifier means has its output connected to output terminals, where a first feedback circuit is connected from the output terminal to an error amplifier, which error amplifier is connected to an input at a control circuit, which output is connected over driver means to the input of the semiconductor switches, where the apparatus further comprises a second feedback circuit where the second feedback circuit is leading a signal from the first coil to an input terminal.
2. Description of Related Art
U.S. Patent Application Publication 2003/043599 describes a DC-DC converter, a regulation method for a DC-DC converter and a switched-mode power supply are proposed. The DC-DC converter comprises an inverter and a primary-side circuit with a transformer whose secondary-side voltage is rectified by at least one rectifier for generating an output DC voltage. To avoid an asymmetrical load, which is in particular exhibited by a different load of the rectifier elements (power semiconductors), an electrical magnitude of the DC-DC converter is measured. This magnitude may, for example, be a primary-side current, a primary-side voltage at a capacitance, or a secondary-side, rectified voltage. From the measurement of the magnitude, a parameter for the symmetry deviation is calculated for which different symmetry measuring methods are proposed. A symmetry regulation arrangement utilizes the drive of the inverter, for example, the duty cycle of the pulse width modulated voltage produced by the inverter to minimize the parameter for the symmetry deviation. This achieves an even distribution of the power over the secondary rectifier elements.
The above mentioned document describes regulation of the switching frequency to keep it clearly above the resonant frequency of the resonant arrangement. The circuit is operating in frequency mode, which leads to a lack of linearity in the relationship between output voltage and output power. The focus in the above mentioned patent application is to avoid an asymmetrical load of the rectifying components, whereas the actual invention is to optimize the output stability by linearizing the feedback.
U.S. Pat. No. 4,935,857 describes a DC to DC series-parallel resonant converter (10) having a plurality of switches (Q1-Q4) which are switched alternatively between on and off states to cause electrical current to flow alternatively in first and second directions through a series-resonant circuit (60) including a variable frequency ramp generator (28) having a reset input (R) for causing an output ramp signal produced at an output to drop to zero in response to each reset signal; a comparator (30) having an input coupled to the output of the ramp signal generator, a second input for controlling the output DC voltage of the series-parallel resonant circuit and an output which changes level each time the ramp signal reaches the magnitude of the second input; a bistable circuit (32) having first and second outputs (Q, Q) for respectively outputting first and second signals, the output signals changing in response to a change in the output signal of the comparator coupled to the input; a pulse generator (26), coupled to the series-parallel resonant circuit for producing an output pulse train with an output pulse occurring each time the flow of current through the series-resonant circuit changes from one of the first and second directions to another of the first and second directions, the output pulses being applied to the reset input of the variable frequency ramp generator to regulate the frequency of the output ramp signal.
The focus in the above mentioned patent is to assure that the switching frequency of the converter is held above the resonance frequency of the serial/parallel converter.
In an apparatus as described in the opening paragraph, it is known to use an integrated circuit L6598 or the like. This integrated circuit comprises a current controlled oscillator which output is connected over driver means to two inverse output terminals, which are directly connectable to the input of semiconductor switches. The oscillator part in L6598 is also connected to the outside through a connecting terminal where this terminal is connected to an external capacitor that together with two internal current controlled current generators set the frequency. An input signal at the integrated circuit is so connected that changes in current through this terminal lead to control of the frequency.
It is achieved that the voltage over the connected capacitor changes in a linear way between two voltage situations. Each time the charge of the capacitor changes its sign in charge current, the oscillator changes its output from a first to a second value, which over the drivers activates and/or deactivates the semiconductor switches. A feedback from the power output is used to control the size of the current used to charge or discharge the capacitor, and thereby, to a change in the frequency of an oscillating system form by extern components. In normal operation, the oscillating frequency oscillates over the resonance frequency of the resonant DC-DC converter, and the first feedback signal leads to a frequency change to a lower frequency nearer the resonance frequency if a higher load is needed. For normal series resonant converter function working above resonant with frequency control, see FIG. 5a. 
When using resonant converters close to resonance frequency, the power gain in the DC-DC converter is highly unlinear with gives big problems in design of the first feedback loop.